The present invention relates to a process for making disposable absorbent articles such as sanitary napkins, panty liners, catamenials, incontinence inserts, and diapers for adults or babies. In particular the present invention relates to a process for the manufacture of disposable absorbent articles comprising two component materials which are adhered to each other, which utilises an improved gravure roll printing process to provide the adhesive.
Absorbent articles such as sanitary napkins, panty liners, catamenials, incontinence inserts and diapers for adults or babies are commonly provided with an adhesive on their garment-facing surface to attach them during their usage period to a garment of the user. In particular sanitary napkins and panty liners are commonly provided with a pressure sensitive, hotmelt, adhesive which attaches to the undergarment of the wearer and thereby improves fit and comfort of the product for the wearer, called panty fastening adhesive. These adhesives are typically covered with a release paper prior to use.
More generally absorbent articles are provided with adhesive areas in order to combine the components which ultimately make up part or the whole of the absorbent article. In particular, multi-layer structures forming the topsheet, core or backsheet are often combined by adhesives called construction adhesives. The combining of the topsheet, the core and the backsheet to each other also can be accomplished by adhesives.
Typically these products are made by high speed machinery. The machinery includes equipment which adds the adhesive in a very fast and efficient manner, ensuring consistency of the absorbent products over large quantities thereof.
A common way of providing an adhesive is by application by slot-coating or spraying of the adhesive onto a continuously conveyed thread of material. The surface of this material, coated with adhesive, is then joined to another material forming all or part of the disposable absorbent article. This adhesive application method allows adhesive application with contact between the apparatus and the material to be coated, in particular for thermoplastic films or nonwoven materials often used in disposable absorbent articles which are very heat sensitive and can be incompatible with adhesives which require high melting temperatures.
An alternative method of adhesive application to material used in absorbent articles is a screen printing method. In the screen printing method, an adhesive is applied to the inside of a rotating roll having apertures. This roll, also called a screen, is contacted with the continuous conveyed thread of material such that adhesive is transferred through the holes of the material. In this case the screen is indirectly heated using infra-red radiation whereby the screen and to some extent the adhesive is heated.
Another alternative application method for adhesive is to render one of the materials used in the production of absorbent articles adhesive. This can be carried out in a separate step independent and long before use of the material in the article. For example adhesive tape or film, as used on baby diapers, have one adhesive surface and one non-adhesive surface. The adhesive surface is not firmly attached (or at least releasably attached) to the non-adhesive surface by winding the tape or film up into a roll. Upon use of the material the roll is then unwound to present the adhesive surface to the material to which it is intended to attach the tape or film.
For example U.S. Pat. No. 5,064,492 provides in this manner an impermeable film onto which a particular adhesive, which is said to be non-blocking below about 43xc2x0 C. (110xc2x0 F.), is printed. The adhesive is provided to the film by a patterned printing roll and a cooled counter roll or a series of such printing units if patterns in the adhesive are desired. The winding of the adhesive coated film is conducted below the blocking temperature.
This method of providing pre-coated film adds multiple complexities to logistics, storage and final manufacturing of disposable articles. Even accidental heating of any portion of the pre-coated roll would render it useless (self adhesion). Also all the benefits of the positioning accuracy of adhesive printing are lost in the alignment difficulties associated with film unwinding. Furthermore, the adhesive has to be heated on the film to return it to a state of adhesivity such. that added energy consumption is paired with a probability of melting through the underlying film material. Also this heating of the adhesive causes degradation due to the added heating/cooling cycle.
One common drawback of all the above mentioned adhesive application processes is their inflexibility, inaccuracy relative to the shape of the adhesive to be applied and that they essentially can only provide the adhesive continuously. Alternatively, on/off systems for coating or spraying not only have the drawback of delay in their systems response but also generally the problems associated with accelerating and decelerating mass streams. For hollow drum screen printing it is possible to create a pattern in the screen which would allow to create adhesive patterns. However, this is limited in that the screen has to provide a fairly even distribution of adhesive application sites (holes) in order to exclude temperature variations resulting in unstable application processes. Also adhesive screen printing cannot provide an even, full surface adhesive coverage due to the maximum apertured dimensions and total open area of such a screen in respect to its stability.
Yet another alternative manner that can be utilised to apply adhesive to a surface requires the utilisation of printing rolls. Such rolls are described in for example EP 745 433 and U.S. Pat. No. 5,417,789. The latter discloses a continuous process for the manufacture of absorbent pads which requires the application of an adhesive to a barrier layer""s outer surface utilising a rotary roll print process. Similarly, U.S. Pat. No. 3,327,708 discloses a laminated non woven web wherein a lightweight cellulose wadding web is bound to a web of synthetic fibres by printing adhesive in a regular open pattern onto the wadding. Likewise GB 2 134 420 and WO 94/29524 describe modified adhesive transfer cylinders to apply adhesive to a continuous web.
However, whilst the utilisation of roll printing in principle also allows for the application of adhesives in patterns on surfaces, the process still has a number of problems associated with it. Typically, the print roll is continuously supplied with adhesive from an adhesive dip bath into which the roll is partially submerged and in which it is rotated. Naturally, the adhesive needs to be supplied in a large excess to allow the print roll to rotate through the adhesive bath and become coated with the adhesive. This of course requires a large amount of energy to be expended particularly in order to maintain the bath and adhesive at the required temperature. Moreover, the rotation of the roll within the bath causes the formation of air bubbles within the adhesive bath which results in the formation of foam. The foam is transferred to the roll and thereby results in the uneven distribution of the adhesive on the roll and consequently onto the substrate, even after scraping. Furthermore, the foam also collects on the scraper itself and is not readily removed therefrom whilst the process is operational. As a result, the process cannot be operated continuously, but, requires regular interruption in order to remove the excess adhesive collected at the scraper.
Yet another problem with the current roll printing process is that the amount and distribution of adhesive which is deposited from the print roll onto the substrate is extremely difficult to control and typically only about 30% of the adhesive contained on the roll is transferred resulting in a highly inefficient process. The amount of adhesive deposited onto the substrate is dependent upon, amongst others, the cohesive properties of the adhesive itself and the temperature at which the adhesive is supplied. In particular, cohesive adhesives tend to exhibit a phenomenon known as stringing during the transfer of the adhesive from the roll to the substrate surface. However it is not currently possible to predict at which point the adhesive will break and thus exactly how much adhesive will be deposited onto the substrate. This results in an irregular application of the adhesive to the surface, in addition to contamination of the adhesive pattern itself.
This problem is further exacerbated when the printing process is operating at low temperatures. However, whilst desirable in terms of improved uniform distribution of the adhesive onto the roll and hence the substrate, increasing the operating temperatures of such printing processes is not usually feasible, because as previously mentioned the components of the disposable absorbent articles onto which the adhesives are applied, are typically temperature sensitive and hence the process needs to be operated below a certain maximum temperature.
Hence, there still exists a need to provide an improved adhesive application method to substrate components of disposable absorbent articles which addresses the aforementioned problems, whilst providing the adhesive in a continuous process, which is also accurate and efficient. Moreover there is also a need to provide an application method which preferably allows the provision of adhesive area shape designs so far not possible at the production speeds typical for disposable absorbent articles.
It has now been surprisingly found that these problems can be addressed by the direct application of the adhesive onto the roll printing by utilisation of techniques such as slot coating, whereby the adhesive is supplied onto the print roll such that the gravures are not completely filled.
The present invention relates to a process for providing a disposable absorbent article comprising a first component and a second component material which are joined by adhesive. The first component comprises a first surface and the second component comprises a second surface. The process comprises the steps of;
a) providing said first component in a machine direction,
b) providing a gravure printing roll with an adhesive
c) contacting said first surface of said first component with said printing roll and transferring at least a portion of said adhesive from said roll to said first surface
d) providing said second component and contacting said first surface of said first component with said second surface of said second component, characterised in that; the gravures of said printing roll are filled less than 85% with said adhesive.
The gravure roll printing step of the process according to the present invention is preferably conducted by a rotating gravure printing roll for transporting the adhesive. The printing roll rotates such that during its 360 path it has a receiving rotation path and a delivery rotation path. The printing roll receives a layer of the adhesive in a receiving portion of its rotation path having an average thickness less than the maximum depth of the gravures and delivers adhesive to the first surface in the delivering portion of the rotation path of the printing roll. The adhesive is typically a hot melt adhesive which is solid or in a semi-solid plastic state at temperatures at or below the usage temperature of the disposable absorbent product for which the adhesive is utilised. The adhesive is applied in a liquefied state, typically achieved by melting, utilising a slot coater. Preferably the slot coater is provided with an adhesive guidance means to provide a path for the adhesive from its exit from the slot coater to the surface of the gravure print roll.
Preferably, the adhesive is applied to the printing roll at a temperature Ta which is greater than, preferably at least 5xc2x0 C. greater than the temperature of the print roll Tr. More preferably the adhesive has a minimum application temperature Ta of 110xc2x0 C. or more.
The adhesive area can provide a releasable attachment means such as is typical between a release paper and a panty fastening adhesive. However, the current process is not limited to such adhesive application but can also provides a permanent connection between the first and the second components.